Sensors 2010, 10, 994-1011; doi:10.3390/s100100994 OPEN ACCESS sensors ISSN 1424-8220 www.mdpi.com/journal/sensors Article A Novel Bioinspired PVDF Micro/Nano Hair Receptor for a Robot Sensing System Fei Li 1, Weiting Liu 1,*, Cesare Stefanini 2, Xin Fu 1 and Paolo Dario 2 1 The State Key Lab of Fluid Power Transmission and Control, Zhejiang University, Zhejiang Province, Hangzhou, 310027, China; E-Mails: [email protected] (F.L.); [email protected] (X.F.) 2 CRIM Lab, Polo Sant'Anna Valdera,Viale Rinaldo Piaggio 34, 56025, Pontedera (Pisa), Italy; E-Mails: [email protected] (C.S.); [email protected] (P.D.) * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +86-571-8795-2226; Fax: +86-571-8795-2226. Received: 1 December 2009; in revised form: 15 December 2009 / Accepted: 11 January 2010 / Published: 26 January 2010 Abstract: This paper describes the concept and design of a novel artificial hair receptor for the sensing system of micro intelligent robots such as a cricket-like jumping mini robot. The concept is inspired from the natural hair receptor of animals, also called cilium or filiform hair by different research groups, which is usually used as a vibration receptor or a flow detector by insects, mammals and fishes. The suspended fiber model is firstly built and the influence of scaling down is analyzed theoretically. The design of this artificial hair receptor is based on aligned suspended PVDF (polyvinylidene fluoride) fibers, manufactures with a novel method called thermo-direct drawing technique, and aligned suspended submicron diameter fibers are thus successfully fabricated on a flexible Kapton. In the post process step, some key problems such as separated electrodes deposition along with the fiber drawing direction and poling of micro/nano fibers to impart them with good piezoeffective activity have been presented. The preliminary validation experiments show that the artificial hair receptor has a reliable response with good sensibility to external pressure variation and, medium flow as well as its prospects in the application on sensing system of mini/micro bio-robots. Keywords: artificial hair receptor; micro/nano fiber fabrication; aligned PVDF micro/nano fiber; thermo-direct drawing Sensors 2010, 10 995 1. Introduction Focusing on mini/micro autonomous robots, lessons from Nature often provide us inspiration for useful solutions and suggest ingenious designs for small bio-robots. In the past, many different bioinspired robot prototypes have been built in the search for efficient and stable solutions for robot gaits. Among them, legged locomotion is preferred by most designers, rather than the simpler wheeled designs, because of its high efficiency, low energy consumption and better stability on un-structured or tough terrains [1] achievable with biped [2-4], quadruped [5,6], to hexapod [7,8] and even octopod configurations [9]. Based on this consideration, we designed our jumping robot prototypes GRILLO II & III (Figure 1a,b) inspired by the small jumping insects known as leafhoppers. Thanks to scale effects and the characteristics of jumping locomotion, this jumping-robot, as a millimeter-sized mobile robot prototype with integrated power supply offers a lot of advantages because of its lesser energy consumption and better practicability [10]. Figure 1. Jumping robot prototypes; (a) GRILLO II has a bioinspired leg design, it shows a good jumping performance but an unstable flight after takeoff [10], (b) prototype GRILLO III is optimized by mass center readjustment and the optimization of the passive forelegs in order to improve the stability during jumping, flight, and landing. After finishing the mechanical design, the work is now focused on the feasibility of the sensing and control systems in order to improve the jumping stability (e.g., wing control during gliding) and to make the robot react to the environment (e.g., flow turbulence detection). Still learning from the natural world, we start from the investigation of the hair cell sensory receptors, which perform as primary mechano-transducers in both the auditory system and the vestibular system of vertebrates. In mammals, the hair cell receptors are located in the cochlea and play an important role in the perception of sound, while in fishes and amphibians, they are located within the lateral line organ to detect the surrounding water motion. Hair cells possess a characteristic organelle which consists of tens of hair-like stereo-cilia. So-called hair bundles are able to pivot around their base when a force is applied to the tips [11-16]. Sensors 2010, 10 996 Sensory hairs widely exist in the natural world (Figure 2). For arthropods, high performance detection systems composed of mechano-receptive cuticular hairs are evolved to sense the slightest air displacement around them, such as that generated by approaching predators. A cricket’s abdomen is covered with mechano-receptive cerci which are sensitive to those slight air currents generated by a wasp’s wings or a toad’s tongue. Such sensory hairs alert the insects when a predator is sneaking around them, and give them a chance to escape from predation [13,17]. The adult tropical wandering spider (Cupiennius salei) has hundreds of trichobothria on its ambulatorial legs and pedipalps ranging from 20 to 1,500 μm in diameter. It was also found those sensory hairs in different length are able to mechanically couple with different frequencies and receive the medium vibration generated by flying insects. Figure 2. Hair cell receptor existing in many animals; (a, b, d) cricket cerci [17], (c) a spider tarsus [18], (e) bat hearing system [19], (f) goldfish (Carassius auratus) lateral line system [20]. Various artificial hair cell (AHC) prototypes based on piezo-silicon cantilever structures have been developed in the past. The feasibility of fabricating an AHC on a silicon substrate through normal lithographic fabrication and the PDMA (plastic deformation magnetic assembly) method was demonstrated [14,21]. Polymer AHC cilia were obtained by patterning SU8 cylinders on silicon piezo-resistive sensors [22]. An all-polymer AHC was fabricated by depositing carbon-impregnated polyurethane force sensitive resistors (FSRs) at the base of vertical cantilever polyurethane cilia. The FSRs transduced the motion of cilium into resistance changes [23]. Another type of interesting high aspect ratio SU8 sensory hairs on a silicon nitride membrane based on capacitive change measurements was presented in [15,24]. The SU8 hairs, up to 1 mm long, underwent a deflection due Sensors 2010, 10 997 to flow momentum, which led to basilar disc rotation and change in the capacitance between the nitride membrane and the substrate. The artificial hair receptors mentioned above all suffer from several drawbacks. First, the fragility of substrate materials limits their application in microsystems and makes them unpractical in the engineering field. As an example, micro bio-robot design frequently involves curved surfaces on which those fragile rigid substrate materials are not easily applied. Second, all these artificial hair prototypes are used to transduce force/momentum to their respective sensitive elements (piezo-resistive or capacitive) instead of being sensitive themselves. That means some other accessories besides the sensory hair itself are necessary, thus leading to an additional fabrication burden and a complicated structure which causes additional difficulties in miniaturization and increases the costs. Furthermore, the aspect ratio of the polymer artificial hairs made by mold techniques and lithographic patterning (usually no more than 20) is not enough compared with its animal counterparts. In this paper the authors address a novel artificial hair receptor (PVDF micro/nano fiber) being sensitive itself, and placed on a flexible substrate with a high aspect ratio and small size, similar to its animal counterpart. This artificial hair is fabricated by a direct fiber drawing technique, by using a micro-syringe pump to push a PVDF solution through a glass micropipette and draw on a substrate. 2. PVDF Hair Receptor Modeling 2.1. A biological Model of Hair Receptor The mechanism, morphology, and modeling of the hair cell type mechano-receptor system such as cricket cercal wind receptors have been unveiled thanks to biological research [12]. The hair is modeled as an inverted pendulum (Figure 3). Figure 3. Inverted pendulum model of hair cell (Shimozawa et al. [12]). Sensors 2010, 10 998 It can be described by a second-order mechanical system which is determined by the spring stiffness S, the momentum of inertia I and the torsion resistance R. For the angular momentum: d 2θ (t) dθ (t) I + R + S θ(t) = N(t) dt 2 dt (1) The hair is deflected by the drag force on the air shaft due to the airflow surrounding the cercus. The total external torque N(t) can be calculated by the integration of the drag force along the hair shaft: L N(t) = 0 F(y,t) ⋅ y ⋅ dy ∫0 (2) 2.2. A Simplified Model for PVDF Artificial Hair Receptor Even though PVDF has shown good mechanical and physical performance in some past research, it is still difficult to entirely duplicate the structure of a biological hair receptor (a vertical free-fix structure) due to the limitations in drawing fabrication and post process techniques. Considering the convenience in our drawing method and the feasibility in electrodes fabrication, a fix-fix structure is selected at the first step. By improving the techniques of fabrication and fiber post-processes, an entirely biological hair structure will be viable in our future work. The PVDF fiber generates the charges on the electrodes while deformed by the flow based on the piezoelectric activity. The generated charge density is: Q Q D = = = d3h X (3) Ae πrL where D is the charge density; Q is the charge; Ae is the effective electrode area; r is the radius of hair fiber; L is the length of the artificial hair (aligned micro/nano PVDF fiber); d3h is the piezoelectric coefficient constant which is the combination of d31, d32 and d33 in our case of in fiber drawing direction, transversal and frontal section direction respectively; X is the stress.